1. Field of the Invention
The present invention relates to an ultrasonic test equipment capable of performing flaw detection in a wide range of an object to be tested, and an evaluation method for evaluating the tested result.
2. Related Art
Ultrasonic testing is a technique for nondestructively confirming soundness of a surface and an inner portion of a structural material that is an object to be tested, and is an indispensable testing technique in various fields. Particularly, in recent years, there is a demand for testing of a spot where ultrasonic testing has never been required in order to ensure safety of a plant structural object or the like. According to such demand, an improvement in efficiency as well as reliability is being required in the ultrasonic testing.
Radiographic testing (RT) and ultrasonic testing (UT) are listed up as effective techniques for accurate sizing of a volume defect existing in a structural object. The RT is effectively used for detecting a defect with a volume, such as a hole, that exists in a structural object, but not suitable for detecting a defect with no volume, such as a crack and a peel-off. Since the RT includes testing steps such as exposure and development, the improvement in efficiency is limited.
On the other hand, the UT is highly applicable to detection of not only a hole within a structural object, but also a plane defect such as a crack and a peel-off. A test result can be also acquired in real time.
The UT generally uses a monocular probe. Recently, phased array ultrasonic testing (PAUT) technology capable of forming any waveform by emitting ultrasonic waves at shifted timings (delay time) from a plurality of small piezoelectric elements arranged in an array probe becomes practically usable. The UT is thus applied to more and more fields. Among the testing techniques for confirming the soundness of a surface and an inner portion of a structural object, the UT is considered to be most suitable for improving the efficiency.
Improving the efficiency in the testing technique means a decrease in test time in total. The decrease in test time is achieved by decreasing a test time per measurement point, and increasing a test range per measurement point. The test time per measurement point depends on a repetition frequency and a signal processing speed of a flaw detection apparatus, and a scanning speed of a probe. The flaw detection apparatus generally has a repetition frequency of 1 kHz or more and also has a signal processing speed according to the repetition frequency. The scanning speed of the probe largely differs depending on an object to be tested or a testing method. It is thus difficult to discuss an increase in the scanning speed in general terms.
The test range per measurement point that involves the improvement in the efficiency in the testing technique is now described. The ultrasonic testing (UT) utilizes a phenomenon of reflection or diffraction caused by an ultrasonic wave entering a defect or the like. In the UT, it is essential that the entering ultrasonic wave and a signal derived from the defect can be temporally or spatially decomposed. The UT means that a frequency band of an order where directionality can be maintained and the defect can be evaluated by use of temporal information is used (the UT uses a frequency band of about 0.5 MHz to 10 MHz for a typical structural material).
In this case, a range tested at a time in the UT depends on the size of an ultrasonic probe. A monocular probe generally has a maximum diameter of 2 inches. If the probe size is simply increased, its resolution is deteriorated, and thus, the increase in the probe to a larger size is not suitable for practical use in flaw detection of a structural object or the like.
In the PAUT, when the number of sensors is endlessly increased, a test range is also increased. However, the array probe has a larger volume, and thus, the handling thereof becomes difficult. It also becomes necessary to process numerous signals, to place a larger load on hardware such as an ultrasonic flaw detector. The area of the test range may be made larger by increasing one channel of each element of the array probe. However, it becomes difficult to control a beam when the elements are spaced at a pitch more than half the wavelength. It is thus not practical to increase one channel of each element.
Accordingly, even in the PAUT, the range tested at a time is limited to an area of several thousand mm2. The increase in the test range has a limit in the PAUT itself.
In addition to the approaches of increasing the number of sensors and increasing the sensor diameter, there are provided, as a method for increasing the test range of a structural object, a method of using a wave in a different mode from a volume wave, such as a longitudinal wave and a traverse wave, used in the above ultrasonic flaw detection, and a method of simply reducing a frequency to increase a range to which a volume wave reaches.
Flaw detection using a guided wave or a surface wave is a representative example of the former ultrasonic flaw detection. Basically, flaw detection using a surface wave is targeted on only a defect existing in an object surface. Therefore, information regarding a defect inside a structural material or a crack depth cannot be obtained. Although flaw detection using a guided wave basically uses a variety of waves ranging from a plate wave such as a Lamb wave to a surface wave, a defect existing at a certain depth in a thick plate that is an object to be tested cannot be effectively detected.
By using the latter method of reducing a frequency to increase the volume wave reaching range, ultrasonic waves can be caused to propagate through an inner portion of a structural material that is an object to be tested as well as a surface of the structural material. However, defect detection sensitivity is deteriorated along with the decrease in frequency.
Thus, a nonlinear frequency component (response of 2f, 3f, and so on, up to nf, or f/2, f/3, and so on, up to f/n to an incident frequency f) generated from a crack portion by increasing a displacement amplitude of an ultrasonic wave and thereby inducing an opening and closing behavior in the crack portion has attracted attention. The nonlinear component is generated only from a defect in a structural material such as a crack. Thus, the nonlinear component could be applied to accurate detection or evaluation of a defect, material deterioration measurement, or the like. An ultrasonic testing technique for nondestructively detecting a defect in a contact interface of an object to be tested (a structural object) has been disclosed in Patent Document 1 (Japanese Patent Laid-Open No. 2004-340807) or the like. An ultrasonic testing technique for enabling accurate detection or sizing of a closed crack that cannot be detected in normal ultrasonic testing by use of a nonlinear ultrasonic wave is disclosed in Patent Document 2 (Japanese Patent Laid-Open No 2005-315636).
The ultrasonic testing technique disclosed in Patent Document 1 is a testing technique for qualitatively detecting a micro-defect in a contact interface of a structural material (object). The ultrasonic testing technique is effective only in detecting the defect in a contact surface of a structural material, and is not used for quantitative evaluation of a defect size.
In the ultrasonic testing technique disclosed in Patent Document 2, depth information of a defect is acquired and evaluated by using a frequency hand in use and temporal axis information. In terms of a test range, it is difficult to increase the test range. The ultrasonic testing technique cannot detect a flaw in a wide range as in the conventional ultrasonic testing.